In response to the RFA "Diffusion of HIV Infection through Sexual Risk Behaviors of Drug Users", a team of established researchers from Yale University, the Hispanic Health Council, the Biomedical Center in St. Petersburg, Russia, and Imperial College, London have established a consortium. Together, they will participate as a single entity in the Sexual Acquisition and Transmission of HIV Cooperation Agreement Program (SATH-CAP). The multidisciplinary team will employ the methodologies of behavioral and molecular epidemiology, medical sociology and anthropology, and mathematical modeling to investigate the behaviors of injection drug users and their sex partners in two locations - Connecticut and St. Petersburg. These two sites have been chosen since they differ in terms of the stage of the HIV-1 epidemic among injectors. In Connecticut, the epidemic appears to be under control, with stable or declining prevalence; in St. Petersburg, the epidemic appears to be on the increase, with high incidence and rising prevalence. This application has four specific aims: 1) to establish a multidisciplinary team based on our competences and expertise to investigate the sexual and syringeborne transmission of HIV and other pathogens among populations of drug users and from drug users to the broader population; 2) to employ a multidisciplinary approach to investigate the dynamics of sexual and syringe-borne disease transmission in cohorts of injection drug users and their sexual partners in the two locations; 3) to trace sexual transmitted HIV-1 infections in St. Petersburg back to injectors to investigate the expansion of the epidemic from one concentrated in injectors to a more generalized one; and 4) to develop mathematical models and projections of disease spread in these populations that are informed by the quantitative, qualitative, and biological data.